Circadian rhythms and environmental lighting regulate a number of endocrine and behavioral functions. Dispersed chick pineal cells remain rhythmic and responsive to light in culture. Changes in cell volume induced by hypertonic solutions mimic the effects of light on the underlying pacemaker, and those induced by hypotonic solutions mimic the effects of darkness and ouabain. Vitamin A depletion reduces the size of the acute effect of light on melatonin output but not its phase shifting effect. Addition of retinaldehyde enhances the acute effect of light but not its entraining effect. These results indicate that two different photopigments mediate the acute and entraining effects on light.